I posted mine on Craigslist and sold it in an hour. I met the guy at the Apple store so he could check it out and activate it, and he gave me cash. Don't jump at the first lowball offer, these things are selling for retail at the least.
I second craigslist. It should be a fairly easy sale, just remember to meet up in a high traffic area (coffee house, etc) for safety. CL is how I flipped my kindle once I took the iPad plunge.